Preparation and Performance of Pyrochlore-Type Oxygen Catalyst for Metal Hydride/Air Secondary Battery

Abstract

A metal hydride (MH)/air secondary battery consists of a gas diffusion electrode as the positive electrode, a hydrogen storage alloy as the negative electrode, and a KOH solution as the electrolyte [1]. Oxygen in air is reduced to produce water at the positive electrode during discharge, so that the MH/air secondary battery has no active mass stored in the positive electrode and the weight and volume of the positive electrode are unnecessary to increase with increasing battery capacity. This unique property is the reason for a high energy density beyond a lithium ion secondary battery. The positive electrode’s reactions are oxygen reduction and evolution during discharge and charge, and the polarization of the positive electrode significantly depends on the bi-functional oxygen catalyst. In our previous work [1], one of pyrochlore type oxides (general formula A2B2O7), Bi2Ir2O7-z, showed a good performance for charge and discharge of the battery, while the overpotential was still high for the oxygen reactions. In this paper, we tried to prepare this type of catalyst by partly substituting iridium with ruthenium, i.e., Bi2[Ir,Ru]2O7-z, and investigated the polarization behaviors for oxygen reduction and evolution in concentrated KOH solutions using rotating disk electrode (RDE).Bi2[Ir,Ru]2O7-z was prepared through co-precipitation in the solution containing H2Ir2Cl6, RuCl3･nH2O, and Bi(NO3)3･5H2O after the addition of NaOH solution, followed by calcination of the precipitate, in which the Ir:Ru ratio and the calcination temperature and time were changed. The obtained oxides were characterized by XRD, SEM, and EDX. Electrochemical measurements were carried out using a three-electrode cell, of which the working electrode was RDE and the electrolyte was 0.1 mol/L KOH solutions. The substrate of RDE was a titanium disk and the oxide particles prepared were dropped and dried on the titanium surface. The anodic polarization measurement by cyclic voltammetry was performed, and the cathodic polarization was also examined by cyclic voltammetry in the solution under N2 or O2 bubbling. The oxygen reduction current was obtained by subtracting the measured current under N2 bubbling from that under O2bubbling.The results by XRD, SEM, and EDX for Bi2Ir2O7-z, Bi2[Ir,Ru]2O7-z (Ir:Ru = 50:50 mol%), and Bi2Ru2O7-z revealed that these oxides contained no by-product and the distribution of the particle size was almost the same, ranging from 20 nm to 80 nm. From the voltammograms obtained with these oxides, the current normalized by the double layer charge, i C, was calculated to compare with each other under the same active surface area. The results clearly indicated that Bi2Ru2O7-z was superior in catalytic activity for oxygen evolution than Bi2Ir2O7-z and the polarization performance of Bi2[Ir,Ru]2O7-z was the same as Bi2Ru2O7-z. Therefore, the substitution of iridium with ruthenium can reduce the overpotential for oxygen evolution, and this trend was also observed for oxygen reduction. In this paper, more results for the effects of the Ir:Ru ratio and the other preparation conditions on the catalytic activity for oxygen reactions will be presented. This work was supported by “Advanced Low Carbon Technology Research and Development Program (ALCA)” of Japan Science and Technology Agency (JST). [1] C. Baba, K. Kawaguchi, and M. Morimitsu, Electrochemistry, 83, 855 (2015).